In Peskin and Schroeder's QFT book, page 189, equation 6.38, how do they get from the first line to the second line?
In particular, I am stuck on the transition from what I perceive to be: $$ k'_\alpha \gamma^\alpha m \gamma^\mu + m k_\alpha \gamma^\alpha \gamma^\mu $$ into: $$ -2m(k+k')^\mu $$
what am I missing?
I thought it might be using the Dirac equation because it works on $u(p)$, but that can't be it since $k\neq p$. Also couldn't figure out how to use the anticommutation relations of the gamma matrices.